The present invention pertains to aqueous cross-linkable polymer compositions for use in coatings and to a process for preparing the same.
Due to increasingly stringent environmental regulations concerning the allowable content of volatile organic compounds in coatings, major efforts have been made to minimize the use of organic co-solvents in water borne coatings based on acrylic binders. However, in water borne coatings that use acrylic polymers as the main binder, the final hardness after curing of an applied coating film is often limited by the amount of organic co-solvent in the coating formulation. The organic solvent is required to lower the film-formation temperature in order to ensure proper coalescence of the polymer particles during the film-forming process. For some coating applications a minimal hardness is required, for example, if a high blocking resistance or scratch resistance is important. The hardness can be increased by raising the glass transition temperature of the acrylic polymer. However, when this coating is applied and dried at ambient temperature this often leads to poor film formation because of the limited amount of coalescing aid allowed. This poor film formation results in reduced gloss, low chemical resistance, and weathering.
Attempts have been made to lower the necessary content of volatile organic compounds in coatings by using two-component coating systems. In order to increase hardness, the two components should cross-link, for which a cross-linker is needed. Such cross-linkers, e.g., polyaziridine or polyisocyanate, often have a toxic nature. Two-component coating systems also show the general drawback that after mixing of the two components the pot life is limited.
A composition and a process of the type according to the opening paragraph are known from PCT patent application WO 95/29944. This publication discloses a process for the production of an aqueous polymer composition for use as a coating with improved hardness and low film forming temperature. Use is made of a cross-linking agent which links the first, water-soluble polymer with the second, hydrophobic polymer. The cross-linking agent reacts by condensation. The lower the water concentration, the more this reaction takes place. It was found that the cross-linking reaction proceeds relatively slowly and begins only after evaporation of all the water in the film. As a result, the early hardness and water resistance are relatively low.
European patent application EP-A 0 587 333 discloses a water-resistant polymer dispersion containing a carboxylated water-soluble polymer which is neutralized and solubilized with, e.g., ammonia. An alkali-insoluble emulsion polymer is prepared in the presence of the water-soluble first polymer. The alkali-insoluble polymer may contain an amine functionality. The composition may comprise a water-soluble polymer with an amine functionality, which serves to neutralize the soluble polymer, as an alternative for the ammonia. During preparation, the two stages are grafted together. Metal ions may be incorporated into the monomer mixture so as to create ionic metal/carboxylate cross-links. The water-soluble stage polymer remains susceptible to bases, which may cause resolubilization when a second layer of paint is applied. The hydrophobic polymer remains thermoplastic and does not cross-link, so that after application the resulting coating film shows a limited hardness.
A common problem with aqueous polymer compositions comprising polymer dispersions with polymers having carboxylic acid-functional groups, is their poor water resistance and poor recoatability, since the water-soluble polymer will easily redissolve in the presence of water or, when a second layer of water-borne paint is applied, under the influence of the present neutralizing base.
The object of the present invention is a polymer composition which can be used as a binder in water borne coatings with a low content of volatile organic compounds, which shows rapid hardness development and good film formation. Another object of the present invention is a process for making such a composition.
This object of the invention is achieved with a composition of the above-described type, wherein the composition further comprises a water-soluble third polymer with an amine functionality.
One embodiment of the invention relates to an aqueous cross-linkable polymer composition comprising:
a) a water-soluble first polymer comprising acid groups, wherein said first polymer is at least partially neutralized with a volatile base;
b) a dispersion of a substantially water-insoluble second polymer, said second polymer comprising carbonyl-functional groups; said second polymer produced by emulsion polymerization in the presence of an aqueous solution of the first polymer;
c) a cross-linking agent capable of undergoing a condensation reaction with the carbonyl-functional groups of the second polymer; and
d) a water-soluble, amine-functional third polymer.
In another embodiment, the invention relates to a process for preparing such an aqueous polymer composition.
More specifically, the present invention pertains to an aqueous cross-linkable polymer composition comprising:
a) a first polymer, which comprises acid groups and is made water-soluble by the addition of a volatile base;
b) a dispersion of a second polymer, which comprises carbonyl-functional groups and which is water-insoluble;
c) a cross-linking agent which can react by condensation with the carbonyl-functional groups of the second polymer; and
d) a third polymer, which is water-soluble and has an amine functionality.
As a result, two different cross-linking reactions will occur after application of the composition according to the invention as a coating. Upon evaporation of the volatile base used to at least partially neutralize the acid groups of the water-soluble first polymer, the amine functionality of the water-soluble third polymer will form ionic bonds with the acid groups of the first polymer. The second cross-linking reaction takes place after evaporation of the water via reaction of the carbonyl-functional groups of the hydrophobic second polymer with the cross-linking agent. The first curing reaction is fast and is responsible for rapid initial hardness development. The second curing reaction is a slow covalent cross-linking and is responsible for the ultimate mechanical properties of the coating. Using a polymer composition according to the present invention, clear as well as pigmented coatings with volatile organic contents below 100 g/liter can be formulated, which have a high hardness, high gloss, high chemical resistance, and good weathering properties after curing.
Coatings comprising a polymer composition according to the present invention can be applied to various substrates, such as metal, wood, paper, cardboard, gypsum, concrete, plastic, etc. Various known application methods may be used, such as brushing, spraying, rolling, dipping, printing, etc. In particular, a polymeric composition according to the present invention can be used in pigmented coating compositions for application directly onto metal (self-priming systems) or as a primer or a top coat for metal, or as a primer, clear coat, or top coat for wood.
The object of the invention is also achieved with a process comprising the steps of:
a) preparing an aqueous solution of an acid-functional first polymer, which is made water-soluble by the addition of a volatile base;
b) preparing by emulsion polymerization in the presence of the aqueous solution of the first polymer, a dispersion of a substantially water-insoluble second polymer comprising carbonyl-functional groups;
c) adding a cross-linking agent which can react with the functional groups of the second polymer,
d) adding a third polymer with an amine functionality to the polymer dispersion after the preparation of the dispersion of the second polymer, optionally after addition of volatile base to the dispersion of the first polymer, the second polymer, and the cross-linking agent, until the pH has a value between 8 and 11.
Alternatively, after the preparation of the water-insoluble second polymer dispersion and the addition of a cross-linking agent which can react with the functional groups of the second polymer, also a third polymer with an amine functionality and a cross-linking agent can be added.
The first polymer may be obtained from ethylenically unsaturated monomers, preferably esters of acrylic and methacrylic acid such as n-butyl (meth)acrylate, methyl (meth)acrylate, ethyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cycloalkyl (meth)acrylates, e.g., isobornyl (meth)acrylate and cyclohexyl (meth)acrylate, or an ethylenically unsaturated compound such as styrene, e.g., normal styrene or substituted styrenes, for instance xcex1-methyl styrene or tert-butyl styrene; vinyl toluene; dienes such as 1,3-butadiene or isoprene, or mixtures thereof. Also vinyl esters, such as vinyl acetate, vinyl alkanoate or their derivatives or mixtures thereof can be used in the monomer composition. Nitriles, such as (meth)acrylonitrile, or olefinically unsaturated halides, such as vinyl chloride, vinylidene chloride, and vinyl fluoride can also be used. The monomer composition also contains unsaturated monomers with acid-functionality. The acid groups of the first polymer are preferably carboxylic acid groups. Unsaturated monomers with one or more carboxylic acid groups are very useful. Optionally, the acid groups are latent as, for example, in maleic anhydride, where the acid-functionality is present in the form of an anhydride group. Also macromonomers comprising one or more carboxylic acid-functional groups can be used. Preferably, monomers such as (meth)acrylic acid are used. Other possible carboxylic acid-functional monomers are oligomerized acrylic acids such as xcex2-carboxyethylacrylate and its higher analogues (commercially available from Rhodia as Sipomer B-CEA), itaconic acid, fumaric acid, maleic acid, citraconic acid, dimeric acrylic acid, or the anhydrides thereof. Besides monomers having carboxylic acid-functionality also monomers possessing a further acid-functional group besides the carboxylic one can be present in the monomer composition, such as sulfonic acid groups, by the copolymerization of monomers such as ethylmethacrylate-2-sulfonic acid or 2-acrylamido-2-methylpropane sulfonic acid, phosphoric or phosphonic groups obtained from monomers such as 2-methyl-2-propenoic acid ethyl-2-phosphate ester (HEMA-phosphate), (1-phenylvinyl)phosphonic acid, or (2-phenylvinyl)-phosphonic acid. Other monomers possessing a further functional group besides the acidic one can also be present in the monomer composition. Examples of such monomers are hydroxy-functional monomers such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, (meth)acrylamide or derivatives of (meth)acrylamide such as N-methylol (meth)acrylamide and diacetone acrylamide. Also the adducts of hydroxy-functional monomers with ethylene or propylene oxide can be present in the monomer composition.
Preferably, the water-soluble first polymer is a copolymer obtainable by emulsion polymerization of styrene, (meth)acrylic acid, and one or more other (co)monomers, e.g., (meth)acrylic esters and/or (meth)acrylamide derivatives. The term xe2x80x9c(meth)acrylxe2x80x9d means xe2x80x9cacrylxe2x80x9d and/or xe2x80x9cmethacryl.xe2x80x9d
Preferably, the water-soluble first polymer further comprises carbonyl-functional groups, which may be keto or aldo groups. As a result, covalent cross-linking will take place with the water-soluble first polymer as well as with the water-insoluble polymer. Preferably, the carbonyl-functional groups are introduced into the water-soluble first polymer by copolymerizing diacetone acrylamide.
The preparation of the water-soluble first polymer can be carried out by means of various techniques, such as solution polymerization, bulk polymerization or emulsion polymerization.
The number average molecular weight of the water-soluble first polymer preferably is between 750 and 15,000. More preferably, the number average molecular weight is between 2,000 and 12,000, most preferably between 5,000 and 9,000. Chain transfer agents, such as mercaptans, can be used to control the number average molecular weight of the polymer.
Preferably, the volatile base is ammonia. Other suitable bases are, for example, volatile amines, such as aminomethylpropanol, dimethylethanolamine or 2-dimethylamino-2-methyl-1-propanol, triethylamine or monoethanolamine. Optionally, a mixture of different volatile bases can be used.
The water-insoluble second polymer may be a homo- or copolymer prepared using esters of acrylic or methacrylic acid such as methyl (meth)acrylate, ethyl (meth)-acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, isobornyl (meth)-acrylate, cyclohexyl (meth)acrylate or an ethylenically unsaturated compound such as styrene, vinyl toluene or xcex1-methyl styrene, o-, m-, and p-methylstyrene, o-, m-, and p-ethylstyrene, p-chlorostyrene, and p-bromostyrene. Also vinyl esters such as vinyl alkanoates (for example the vinyl esters of versatic acids, which are commercially available under the trademark VeoVa 9, VeoVa 10 and VeoVa 11, ex Shell Chemicals), and vinyl acetate and its derivatives; vinyl halides such as vinyl chloride; vinylidene halides such as vinylidene chloride; dienes such as 1,3-butadiene, and isoprene can be used in the monomer composition. The monomer composition can also contain minor amounts of carboxylic acid-functional monomers such as (meth)acrylic acid and monomers possessing a second functional group such as amino or ethylene ureido groups. The carbonyl-functional groups of the hydrophobic second polymer are preferably based on aldo or keto groups for imparting cross-linking through the formation of covalent bonds. Specific monomers that can be used in the copolymerization to introduce these groups include, but are not limited to, (meth)acrolein, diacetone acrylamide, formyl styrol, diacetone acrylate, acetonitrile acrylate, diacetone methacrylate, 2-hydroxypropyl acrylate acetyl acetate, butanediol-1,4 acrylate acetyl acetate, or a vinyl alkyl ketone, e.g., vinyl methyl ketone, vinyl ethyl ketone or vinyl butyl ketone.
Very good results are obtained if both the water-soluble first polymer and the water-insoluble second polymer are prepared by copolymerizing diacetone acrylamide monomer with the other monomers. In this case, the carbonyl-functional groups of the first and the second polymer will be the carbonyl groups of the polymerization product of diacetone acrylamide monomer with the other monomers, such as those mentioned herein.
The hydrophobic second polymer may be, and is preferably, prepared by emulsion polymerization in the presence of an aqueous solution of the water-soluble first polymer. Additional emulsifying agents can be used.
Emulsifying agents that can be used for the emulsion polymerization of the water-soluble first polymer and/or the water-insoluble second polymer are, for example, anionic and/or non-ionic emulsifiers. Anionic emulsifiers include, but are not limited to, alkylethoxylate sulfate and sulfonate, alkylphenolethoxylate sulfate and sulfonate, alkylsulfate and sulfonate, alkylethoxylate phosphates, alkylphenol ethoxylate phosphates, alkyl phosphates, alkylaryl sulfonates, sulfosuccinates, and mixtures thereof. Non-ionic surfactants include, but are not limited to, alkylaryl polyether alcohols, alkylphenol ethoxylates, alkyl ethoxylates, ethylene oxide block copolymers, propylene oxide block copolymers, polyethylene oxide sorbitan fatty acid esters, and mixtures thereof. Preferably, the amount of emulsifying agent used is between 0.3 to 2% by weight, based on the weight of the total amount of monomer. More preferred is the use of an amount of emulsifying agent of 0.3 to 1% by weight. Preferably, no additional emulsifiers are used in the emulsion polymerization of the water-insoluble second polymer if the water-soluble first polymer is also prepared by emulsion polymerization.
In a preferred embodiment of the composition according to the invention, the weight ratio of the first, water-soluble polymer to the second, hydrophobic polymer is between 60/40 and 5/95, more preferably between 35/65 and 15/85.
If the water-soluble first polymer and/or the water-insoluble second polymer are prepared by emulsion polymerization, the polymerization can be initiated with free-radical forming initiators such as alkali persulfate, ammonium persulfate, azo-bis-isobutyronitrile, 4,4xe2x80x2-azo-bis-cyanovaleric acid, organic (hydro)peroxides or peresters, such as tert-butyl hydroperoxide or tert-butyl perpivalate. The radicals can be formed, for instance, by heating the reaction mixture or by the use of a reducing compound, optionally combined with a metal salt. Reducing compounds are sodium pyrosulfite, sodium formaldehyde sulfoxylate, ascorbic acid, and the like. Depending on the initiation system that is used, the reaction temperature generally is between 20 and 95xc2x0 C.
The amine-functional third polymer can be prepared, for example, by ring opening polymerization of heterocyclic monomers, such as ethanimine, 1-propanimine, azetidine, azetidine derivatives, or mixtures thereof. A suitable polyethylene imine that can be used is commercially available under the trademark Lupasol(copyright) FG or Lupasol(copyright) FC ex BASF. Alternatively, the third, amine-functional polymer can be prepared by radical polymerization of xcex1,xcex2-ethylenically unsaturated amine-functional monomers, such as dimethylaminoethyl methacrylate, tert-butyl aminoethyl methacrylate, or mixtures thereof. A third possible method to produce the amine-functional third polymer is by using monomers containing a blocked amine group such as aldimine, ketimine, enamine or, preferably, oxazolidine. Mixtures of monomers containing a blocked amine group may also be used. These compounds can be hydrolyzed into primary and secondary amines. Preferably, the amine-functional third polymer is poly(dimethylaminoethyl methacrylate). It is also preferred that the amine-functional third polymer has a number average molecular weight from 500 to 100,000, more preferably from 1,000 to 80,000. The amount of amine-functional third polymer preferably ranges from 0.1 to 5%, based on the weight of the first and second polymers.
The cross-linking agent can be a hydrazine-functional agent, preferably a hydrazide-functional agent, more preferably a dihydrazide, e.g., a dicarboxylic acid dihydrazide such as oxalic acid dihydrazide, malonic acid dihydrazide, succinic acid dihydrazide, glutaric acid dihydrazide, adipic acid dihydrazide, sebacic acid dihydrazide, maleic acid dihydrazide, fumaric acid dihydrazide, itaconic acid dihydrazide, phthalic acid dihydrazide, or terephthalic acid dihydrazide. Alternatively, the cross-linking agent can be a water-soluble aliphatic dihydrazine, such as ethylene-1,2-dihydrazine, propylene-1,3-dihydrazine, and butylene-1,4-dihydrazine, or a polyamine such as isophorone diamine or 4,7-dioxadecane-1,10-diamine. Preferably, the cross-linking agent is adipic dihydrazide.
If so desired with regard to the end use, the aqueous polymer composition of the present invention may further contain various additives such as dispersants, lubricants, anti-foaming agents, solvents, film formation aids, plasticizers, anti-freezing agents, waxes, preservatives, thickeners, etc.
The polymer composition may be used as a clear varnish or may contain pigments. Examples of pigments suitable for use are metal oxides, such as titanium dioxide or iron oxide, or other inorganic or organic pigments.